Determine name of a JavaScript object instance's class

Question

Imagine a JavaScript "class" Foo:

var Foo = function()
{
};

And an instance of that class:

var foo = new Foo();

Can I obtain the string Foo directly from the instance foo, or is Foo just a transitive variable that cannot be associated with the instance foo after instantiation?

EDIT 1 SLaks suggests using foo.constructor. This gives:

function ()
{
}

This approach works if the function is defined in the form:

function Foo()
{}

...but this might not always be the case.

EDIT 2 Trying skizeey's approach doesn't work either. It is essentially a more complete attempt as Slack's method, but yields an empty string: ""

EDIT 3 I wonder whether this is actually possible somehow. Notice the following transcript from Chrome's JavaScript console:

> var Foo = function() {}
undefined
> var foo = new Foo();
undefined
> foo
Foo

In the last line, Chrome clearly knows that the object is of type Foo. However this may just be a Chrome thing, and not standard JavaScript or even inaccessible from the language itself.

Do you really need the string "Foo"? What are you ultimately trying to do? — I Hate Lazy
– I Hate Lazy, Commented Oct 21, 2012 at 1:54
I'm trying to leverage a convention in my code. I'm using RequireJS and knowing this name would mean I'd know what module the class was defined in, so I could load it again upon resaturating data from local storage. — Drew Noakes
– Drew Noakes, Commented Oct 21, 2012 at 1:57
If you control the constructors in the modules being required, you may just need to write the names in. If you don't, then are you hoping to re-associate the data with its original prototype chain? Or is it merely to know which modules to load? — I Hate Lazy
– I Hate Lazy, Commented Oct 21, 2012 at 2:06
@user1689607, both in a sense. By reloading the correct module, I'll have access to an instance of the class where the name matches the module. Unless someone can find a way to do this, I'll even have to add my own property to each module that fits this pattern, or change the way I declare my constructors. — Drew Noakes
– Drew Noakes, Commented Oct 21, 2012 at 2:10
Generally I'd suggest using named functions for constructors, if for nothing else, it helps in debugging. You can keep using function expressions, but use named expressions. var Foo = function Foo() {...}; Though I'd prefer standard declarations. function Foo(){...} — I Hate Lazy
– I Hate Lazy, Commented Oct 21, 2012 at 2:21

jurassix · Accepted Answer · 2013-10-14 14:15:28Z

4

With named function:

> var Foo = function Foo(){};
> undefined
> var foo = new Foo();
> undefined
> foo.constructor.name
> "Foo"

With unnamed function:

> var Foo = function (){};
> undefined
> var foo = new Foo();
> undefined
> foo.constructor.name
> ""

answered Oct 14, 2013 at 14:15

jurassix

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Comments

Mark Reed · Accepted Answer · 2012-10-21 02:14:16Z

2

There is no way to achieve what you want without modifying the module code.

When you define a function like

var Foo = function() {...}

The function - and any object created using it as a constructor - has no knowledge of the name Foo. You might as well do var x = 0 and then attempt to get the value 0 to tell you about the name x.

answered Oct 21, 2012 at 2:14

Mark Reed

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1 Comment

Drew Noakes Over a year ago

Take a look at my third edit. Somehow Chrome knows the name of the class, even though it was not from a named function expression. I want to know whether this is something that's part of the standard, or just a privilege that Chrome is flaunting.

SLaks · Accepted Answer · 2012-10-21 01:50:17Z

0

You can get the Foo function by from foo.constructor.
However, you cannot associate that function instance with the name of the variable you assign it to.

answered Oct 21, 2012 at 1:50

SLaks

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1 Comment

clentfort Over a year ago

Unless you overwrite the constructor property.

Community · Accepted Answer · 2017-05-23 10:27:46Z

0

From another thread:

var getName = function(object) { 
  var funcNameRegex = /function (.{1,})\(/;
  var results = (funcNameRegex).exec(object.constructor.toString());
  return (results && results.length > 1) ? results[1] : "";
};

You're essentially just grabbing the entire constructor as a string, and then extracting its name.

One caveat though: like what clentfort said on a comment to SLaks' answer, there's no one stopping anyone from overwriting an object's constructor property.

Another caveat: you are also going to have to explicitly name your function, too.

edited May 23, 2017 at 10:27

CommunityBot

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answered Oct 21, 2012 at 2:06

Sal Rahman

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1 Comment

Drew Noakes Over a year ago

it still doesn't work. Try it with the code sample from my question. It just returns an empty string.

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